In recent years, a satellite communication service in which communication between general mobile terminals is performed via a communication satellite has been under study. The communication satellite typically comprises, at a front end, a low-noise amplifier for amplifying, with low noise, a signal emitted by the mobile terminal. The low-noise amplifier is known to deteriorate in performance when receiving a signal of excessive strength. When the strength of the signal received by the low-noise amplifier is excessive, it is necessary to reduce the signal before the low-noise amplifier receives the signal.
In the satellite communication service in particular, strength of a signal received by the low-noise amplifier changes depending on the number of mobile terminals that emit signals to the communication satellite. Therefore, there is a need to control, depending on strength of a reception signal received by the communication satellite, the signal received by the low-noise amplifier.
PTL 1 discloses a receiving high-frequency circuit comprising a protection circuit of a low-noise amplifier, capable of determining that the low-noise amplifier receives an excessive signal, and protecting the low-noise amplifier from the excessive signal. According to the technique described in PTL 1, the low-noise amplifier is operated by two electrodes of a positive electrode and a negative electrode, and when a signal that is determined to have excessive strength is received, one of the electrodes is cut off, whereby the signal received by the low-noise amplifier is reduced.
PTL 2 discloses an excessive input protection circuit capable of easily following a change in strength of a reception signal, and preventing breakage of a low-noise amplifier. The technique described in PTL 2 comprises an excessive input determining circuit for determining the strength of the signal, so that when a signal of excessive input is received, a flow path of the signal is switched, whereby the low-noise amplifier is protected.